The present invention regards a method for compensating the position offset of a capacitive inertial sensor, and a capacitive inertial sensor.
As is known, owing to their reduced size, excellent technical characteristics, high reliability, and low cost, integrated capacitive inertial sensors manufactured using the micromachining technique are progressively laying claim to market segments up to now occupied by conventional inertial sensors. One of the main applications of the above inertial sensors is in the field of airbag systems for motor vehicles as a means for measuring the deceleration to which a motor vehicle is subjected upon impact.
To provide an example, FIG. 1 presents the structure of an integrated rotary inertial sensor of a known type.
The inertial sensor, indicated as a whole by 1, is made of semiconductor material, has a circular structure, and comprises an inner stator 2, which is integral with the die 3 in which the inertial sensor 1 is formed, and an outer rotor 4, which is electrostatically coupled to the stator 2.
The rotor 4 comprises a suspended mass 6 having an annular shape, a plurality of mobile arms 8, extending radially towards the stator 2 from the suspended mass 6, identical to each other and angularly equispaced, and elastic-suspension and anchorage elements 10 (represented schematically as springs) elastically connecting the suspended mass 6 to fixed anchoring and biasing regions 12, through which the suspended mass 6 and the mobile arms 8 are biased (typically at a potential of 1.5 V).
The stator 2 comprises a plurality of pairs of fixed arms 14, 16, one for each mobile arm 8 of the rotor 4, which extend radially with respect to the suspended mass 6 towards the suspended mass 6. They are arranged in such a way that between each pair of fixed arms 14, 16, a corresponding mobile arm 14 of the rotor 4 is arranged and are connected to respective fixed anchoring and biasing regions 18, 20, through which the fixed arms 14, 16 are biased (typically at a potential ranging between 1.5 and 2.2 V).
The fixed arms 14, 16 are connected, via the fixed anchoring and biasing regions 18, 20, to a measuring circuit having the purpose of measuring the acceleration or deceleration to which the inertial sensor 1 is subjected.
In particular, the inertial sensor 1 can be electrically modeled as shown in FIG. 2, i.e., by means of two capacitive elements 21, 22 connected in series, wherein the two outer plates are defined by the fixed arms 14 and 16, respectively, of the stator 2, and the two inner plates are defined by the mobile arms 8 of the rotor 4, which although they are illustrated as being separate, in fact constitute a single plate.
The rotational motion of the rotor 4 determines a modulation in phase opposition of the capacitances of the capacitive elements 21, 22, which should assume, in the absence of acceleration or deceleration applied to the inertial sensor 1, equal values. Consequently, by measuring these capacitances, it is possible to detect the unknown inertial quantity, i.e., the acceleration or deceleration to which the inertial sensor 1 is subjected.
It is also known, however, that, on account of the imperfect configuration of the elastic-suspension and anchoring elements 10 and on account of the residual mechanical stress of the material of which the inertial sensor 1 is made, the rotor 4 is generally affected by a position offset; i.e., the effective zero position of the rotor 4 does not coincide with the nominal zero position envisaged in the design phase.
The position offset consequently gives rise to a corresponding capacitive offset, defined as the difference between the capacitances of the capacitive elements 21, 22 in the absence of acceleration or deceleration, which has an adverse effect on the overall performance of the system comprising the inertial sensor 1 and the corresponding driving and measuring circuitry.
A known technique used for compensating the aforesaid capacitive offset involves the use, within the measuring circuit, of regulatable compensation capacitors, which are connected in parallel to the capacitive elements 21, 22 and have the purpose of compensating the differences which, in the absence of acceleration or deceleration, the capacitances of the said capacitive elements 21, 22 present as compared to the nominal values which they ought to assume in the absence of position offset. In this way, then, even in the presence of a capacitive offset, the equivalent capacitances measured by the measuring circuit in static conditions, i.e., in the absence of acceleration or deceleration, again assume the same value.
This technique presents, however, the drawback of compensating the capacitive offset only under static conditions, i.e., in the absence of acceleration or deceleration applied to the inertial sensor, but not under dynamic conditions, i.e., in the presence of acceleration or deceleration applied to the inertial sensor, and this is typically a cause of errors in the measurement of the unknown inertial quantity.
In fact, after the compensation performed as described above, the rotor 4 continues in any case to assume a zero position that is not the nominal one, and because of the position offset the application of an acceleration or deceleration to the inertial sensor 1 does not bring about any modulation in phase opposition of the capacitances of the capacitive elements 21, 22 that occurs in the absence of position offset, but causes asymmetrical variations of these capacitances which depend both on the direction of rotation of the rotor 4 and on the amount of the position offset; these variations consequently lead to measuring errors.
The present invention provides a method for compensating the offset and a capacitive inertial sensor that is free from the drawbacks of the known art.
According to the disclosed embodiments of the present invention, the inertial sensor is made of semiconductor material and comprises a stator element and a rotor element electrostatically coupled together and an actuator made of semiconductor material coupled to the rotor element and controlled to compensate the position offset of the rotor element with respect to the stator element.
In accordance with another aspect of the present invention, an inertial sensor is provided that includes a sensor element having a stator and a rotor, and an actuator formed on the sensor element, the actuator comprising a fixed arm connected to one of the stator and the rotor and a mobile arm connected to the other of the stator and the rotor, the actuator configured to adjust the positions of the stator and the rotor relative to one another in response to a driving signal.
In accordance with another aspect of this embodiment of the invention, a driver circuit is provided that is coupled to the sensor element and the actuator and configured to determine a position offset of the stator and rotor relative to one another and to generate the driving signal in response thereto.
In accordance with yet another embodiment of the present invention, a method for compensating the position offset of an inertial sensor made of semiconductor material and having a stator element and a rotor element electrostatically coupled together is provided. The method includes moving the rotor element relative to the stator element to compensate for the position offset thereof.
In accordance with another aspect of the method of the present invention, the moving of the rotor includes driving at least one actuator element made of semiconductor material coupled to the rotor element. More particularly, driving includes applying a potential difference between a mobile arm and a fixed arm in the actuator element.